For a long time, there has been a desire for a material having properties between those of cemented carbide and high speed steel. The aim of such a material would be to achieve the positive properties of each type of material, such as the high wear resistance or cemented carbide and the good toughness behavior of high speed steel.
A part of the range of properties between cemented carbide and high speed steel is covered by the material made according to U.S. Pat. No. 4,145,213 which relates to an alloy manufactured by powder metallurgy and comprising 30-70 percent by volume of hard principles in a metallic binder phase. The hard principles are extremely fine-grained having a mean grain size of 0.04-0.70/.mu.m. The binder phase is based on Fe, Ni and/or Co. The hard principles comprise especially carbides, nitrides and/or carbonitrides based on Ti, Zr, Hf, V, Nb, Ta, with additions of essentially Cr, Mo and/or W. Such a material is more like cemented carbide than high speed steel with respect to properties such as cutting material and machinability.
A method of preparing powder of the desired kind is disclosed in U.S. patent application Ser. No. 163,155 filed Feb. 25, 1988 as a continuation of U.S. patent application Ser. No. 906,437, filed Sept. 12, 1986 and now abandoned. However, the manufacturing of final cutting tools from the type of material discussed above can give rise to considerable processing problems. Grinding, for example, involves problems because the material causes wear and also contains so much binder phase that the grinding wheels become clogged leading to burning, etc. These problems have been solved by the techniques disclosed in U.S. Pat. No. 4,618,540, including a compound design which makes manufacturing of complicated tools such as shank end mills possible, in which the positive properties of the hard material such as wear resistance have been combined with the toughness behavior of a steel core material. This design also solved the grinding problems in an economically satisfactory way.
It has now been found, however, that there is a need of a material having a considerably improved wear resistance as a cutting tool material in chipforming machining compared to high speed steel and which is also machinable with conventional cutting tools to manufacture the desired tool. The hard material referred to above is, of course, less suitable in this respect.
Attempts have been made to improve high speed steel by powder metallurgy. Powder metallurgyhhas shown significant advantages over conventional metallurgy which uses large ingots rolled to desired dimensions. By means of powder metallurgy, much greater amounts of carbides can be used in high speed steels than by conventional melt metallurgy. The practical maximum limit for alloying of high speed steels is about 2.3% C, 7% Mo, 6.5% W, 6.5% V and 4% Cr. In addition, there is an upper limit of about 12% for Co after which the material shows extensive brittle behavior. These limits are the practical limits before precipitation of large primary carbides takes place in the melt. Such a material is commercially available and represents an advanced high speed steel with respect to wear resistance. It is built up of well balanced alloying additions and has a controlled mean grain size of 1-2/.mu.m.
It has also been attempted by powder metallurgical techniques to increase the amount of hard principles in `more simple` high speed steels such as type M2 (0.9% C, 4.0% Cr, 5.0% Mo, 6.5% W, 2% V, remainder Fe and normal impurities). In such attempts, a high speed steel powder was prepared by granulation after which additional hard principles in the form of elementary powders such as, for example, pure carbides, preferably TiC, were mixed. Thereafter, the procedure was continued as if no additional hard principles were present, for example, by cold isostatic pressing (CIP)+hot isostatic pressing (HIP)+hot rolling. Such attempts have not been successful because the added hard principles are not uniformly distributed in the material usually forming clods and, in most cases, present as long bands in the working direction. This gives rise to weaknesses in the material being at least as serious as the carbide bands present in conventional high speed steels as a consequence of segregations during the solidification of large ingots. Tools manufactured from such a material are characterized as having not only a more evident brittleness behavior than the powder metallurgical high speed steels discussed above but also an insufficient wear resistance in many applications because large areas are too soft which leads to nonuniform edges and rapid wear in the form of flaws which will undermine the integrity of the material and give rise to total breakdown.
The hard material of U.S. Pat. No. 4,145,213 has a transverse rupture strength corresponding to that of the most high-alloyed high speed steels on the market.
It has now been found that the amount of hard principles in a high speed steel powder can be increased to a desired level by adding said hard material or, by a contrary mode of expression, decrease the amount of hard principles in the hard material by `dilution` with high speed steel powder to obtain the desired advantages, i.e., a material having a considerably improved wear resistance behavior compared to high speed steel but still being machinable by means of turning, milling, drilling, etc. and without obtaining negative properties such as an impaired macro toughness behavior and an uneven distribution of harder and softer parts.
Materials having the above-mentioned properties are particularly desirable when making tools, the manufacture of which involves the removal of large amounts of material and also for tools in which plain hard material is used, e.g., end mills, drills, reamers, hobs, threading tools, etc., in which some of the wear resistance can be sacrificed in order to obtain an improved toughness behavior. As known, no material is complete but each type of material has its particular uses and application areas.